The ocean quahog, Arctica islandica, is a commercially important species on the northwestern Atlantic continental shelf. This species can live over 200 years with ages varying widely per 5-mm size class (shell length), making study of age-related demographics di cult. To determine how the demography of this species differs across its range, including age frequencies, age-at-length relationships, and sex ratios, multiple populations from the Mid-Atlantic Bight and Georges Bank were studied. The age and length frequencies from two populations collected in 2019 from New Jersey, north and south of Hudson Canyon (39.840556 N, 72.821667 W and 39.33 N, 73.545 W, respectively), were analyzed and compared to two northern populations from Long Island (40.09658 N, 73.01057 W) and Georges Bank (40.72767 N, 67.79850 W), previously collected in 2015 and 2017. Sex-speci c differences were also analyzed to determine how said differences in uenced a range of demographic metrics. Though some similarities in recruitment trends exist, likely caused by large-geographic-scale environmental conditions, each site contains a distinct population with distinct demographics. Recruitment is consistent over hundreds of years, but year-to-year and decadal-length variations are apparent. One age-length key may potentially be used to represent Long Island and northern New Jersey, but the remaining sites are signi cantly dissimilar. Sex ratios also vary, potentially caused by differential survival of one sex during certain times in the past. Mortality rates for Georges Bank are distinctly higher compared to the other populations.Based on these results, the use of separate age-length keys would seem necessary for each population and each sex.

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Organisms increase in size over time (age) due to excess assimilation over metabolic (respiration) energy demands. Most organisms reach a maximum size with increasing age as gain and loss balance. The von Bertalanffy length-at-age relationship, which is commonly used in fishery assessment calculations, imposes such a maximum size. However, some fished species, such as ocean quahogs, Arctica islandica, are long lived and continue to grow at old age. The Tanaka age-at-length relationship has continued growth at old age, but is rarely used in stock assessment models. A modified form of the von Bertalanffy model is presented, which mimics the continued growth at old age of the Tanaka model by allowing the growth parameter (K) to decline with age. This form is suitable for inclusion in stock assessment models based on von Bertalanffy. The proposed model matches Tanaka curves with precision appropriate for the scatter of data used to fit the curves. The observations of ocean quahog length at age and growth rate from New Jersey and Georges Bank demonstrate the ability of the modified von Bertalanffy relationship to represent continued growth at old age for this fished species. Simulated data generated with continued growth at old age were fit with the Stock Synthesis model (SS3). Results comparing traditional and modified growth relationships showed that the original von Bertalanffy model can reasonably approximate modest nonasymptotic growth as long as the number of observations is sufficient to constrain the parameter values.

No abstract

The ocean quahog, Arctica islandica, is a commercially important species on the northwestern Atlantic continental shelf. This species can live over 200 years with ages varying widely per 5-mm size class (shell length), making study of age-related demographics difficult. To determine how the demography of this species differs across its range, including age frequencies, age-at-length relationships, and sex ratios, multiple populations from the Mid-Atlantic Bight and Georges Bank were studied. The age and length frequencies from two populations collected in 2019 from New Jersey, north and south of Hudson Canyon (39.840556 N, 72.821667 W and 39.33 N, 73.545 W, respectively), were analyzed and compared to two northern populations from Long Island (40.09658 N, 73.01057 W) and Georges Bank (40.72767 N, 67.79850 W), previously collected in 2015 and 2017. Sex-specific differences were also analyzed to determine how said differences influenced a range of demographic metrics. Though some similarities in recruitment trends exist, likely caused by large-geographic-scale environmental conditions, each site contains a distinct population with distinct demographics. Recruitment is consistent over hundreds of years, but year-to-year and decadal-length variations are apparent. One age-length key may potentially be used to represent Long Island and northern New Jersey, but the remaining sites are significantly dissimilar. Sex ratios also vary, potentially caused by differential survival of one sex during certain times in the past. Mortality rates for Georges Bank are distinctly higher compared to the other populations. Based on these results, the use of separate age-length keys would seem necessary for each population and each sex.